1. Field of the Invention
This invention relates to the preparation of benzoic acid esters. More particularly, this invention relates to a continuous batch process for preparing esters from mono- or dihydric alcohols and benzoic acid that may contain at least trace amounts of at least one of the three isomeric phthalic acids or phthalic anhydride. The resultant benzoic acid esters are at least 99 percent pure and substantially free of phthalic acid esters.
2. Description of the Prior Art
Methods for preparing esters, including those of aromatic acids such as benzoic and the three isomeric phthalic acids are well known. In accordance with one of these methods the carboxylic acid and the alcohol are typically reacted in the presence of a suitable catalyst. Because this type of esterification is usually an equilibrium reaction, to favor formation of the desired ester a stoichiometric excess of the alcohol reactant is typically used and the water formed as a by-product is continuously removed. Strong acids, stannates, and organic titanates are typical catalysts for esterification reactions.
If the boiling point of the final ester is sufficiently low, it can be isolated from the reaction mixture by distillation.
The distillation of esters prepared from 1) aromatic acids such as benzoic and phthalic acids and 2) alcohols containing less than 4 carbon atoms is known. For example, Japanese laid open application 60/94939, published on May 28, 1985 describes a method for purifying esters by treating the initial ester with a solid alkali or a concentrated alkaline solution followed by distillation of the resultant mixture. No conditions for the distillation are reported. The only esters exemplified are mono- and dimethyl phthalate.
Fractional distillation of dimethyl terephthalate at a temperature of 185-7.degree. C. under a pressure of 50 torr is described by P.Zernov et al in Khim. Volokna (1975) (4) 27-8.
Distillation of ethyl 2,3,4,5-tetrafluorobenzoate is described in Japanese laid open application 11/130,734, laid open on May 18, 1999. The ester is prepared from the corresponding acid and alcohol in the presence of a sulfonic acid catalyst, which is subsequently recycled.
Steam distillation of esters prepared by reacting phthalic acid and an alcohol containing from 5 to 12 carbon atoms is described in Russian patent No. 763,322, published on Sep. 15, 1980. Steam distillation yields a mixture of the desired ester together with water, which must be removed as part of the purification procedure. Because removal of the last traces of water from an organic compound is difficult, expensive and time consuming, steam distillation is less than satisfactory when the objective is isolation of a high purity, substantially anhydrous ester directly from a reaction mixture.
The failure of the prior art to teach distillation of esters derived from aromatic acids and alcohols containing more than about 6 carbon atoms in the absence of steam suggests that conventional distillation of these esters is not feasible, particularly on a commercial scale, even under reduced pressures.
Commercially available grades of benzoic acid typically contains at least trace amounts of at least one of the three isomeric phthalic acids and/or phthalic anhydride. One source for these impurities is the presence of one or more of the three isomeric xylenes in the toluene that is oxidized in accordance with one of the more commonly used commercial processes for preparing benzoic acid. Zone refining and distillation have been used to separate benzoic acid from these impurities, however these methods are not cost efficient.
The toxicity to humans of esters derived from phthalic acid and alcohols containing 8 carbon atoms has been investigated by government regulatory agencies. Use of these esters as plasticizers in toys for children under 3 is banned in several European countries.
In the United States, for certain end use applications of the corresponding benzoic acid esters, for example as plasticizers for vinyl polymers such as polyvinyl chloride used as food grade packaging materials, the esters must contain no more than 0.1 percent by weight of the corresponding phthalic acid esters in order to avoid the reporting requirements of federal and state regulatory agencies.
One objective of this invention is to provide a semi-continuous process for preparing substantially pure esters from benzoic acid and either monohydric alcohols containing from 6 to 12 carbon atoms or dihydric alcohols containing from 2 to 8 carbon atoms.